If you've ever ridden on a subway train, you know that there is a lot of energy contained in slowing down and stopping. You feel it as you're jostled into the person next to you. Until now, most transit systems have just lost that energy as waste heat, but a couple of years ago the South Eastern Pennsylvania Transit Authority (SEPTA) in Philadelphia realized that all that energy could be harvested and used and this June, it turned on a regenerative braking system on one of the tracks to capture that energy.

The trains that run on the Market-Frankford line are outfitted with a regenerative braking system much like what hybrid and electric cars use to feed braking energy back to the battery, except the trains feed that energy to the electric third rail where it is then stored in system of 4,000 30 Ah nickel cobalt aluminum batteries.

Now this is where this energy harvesting plan gets really cool. SEPTA has partnered with smart grid technology company Viridity to set up a system for selling that stored power back to the grid during peak demand hours. There's a lot of talk about large-scale battery storage for renewable sources of energy like solar and wind that would allow the electricity from those sources to be more easily incorporated into the grid, but that idea can just as easily be used in a situation like this to store energy that is otherwise being wasted. And when you couple that with smart grid technologies that can automate the delivery of that energy to the grid, then you're not only talking about clean energy, but reliable energy too.

SEPTA expects to save $190,000 in energy costs from reusing the harvested braking energy to power its trains as well as seeing less wear and tear on the trains' brakes. By selling additional electricity to the grid, it could make another $75,000 - $250,000. That's just for one line; the transit authority has plans to expand the system to another part of Philadelphia soon.

Viridity says that 95 percent of the energy captured from the system can be reused, but right now the system is only able to capture 1.5 MW at a time due to funding limitations. So, while there is still a lot of braking energy being wasted, this project shows the great energy harvesting potential for SEPTA and transit systems around the world, especially if smart grid technologies are used to incorporate that energy during times of peak demand.